— There are between 92 million and 180 million great tits in cities and natural habitats across Europe, according to BirdLife International (Image: Current Biology/Slabberkoorn)
This sonogram shows a song's variation in pitch (frequency) with time for great tits singing in the city of Brussels (left) and in the nearby Rivière forest (right) (Image: Current Biology/Slabberkoorn)
Noise pollution in the city (top line) tends to be lower pitched than noise in forests (bottom line): the sonograms show songs against increasing levels of noise. A – no noise, C - high urban noise, F – high forest noise (Image: Current Biology/Slabberkoorn)
Entire populations of birds across Europe are adapting their song to be better heard above the din of the city.
It is well-established that some birds are able to modulate their songs to adapt to different environments. In 2004, for instance, researchers showed that individual nightingales made their songs very much louder so they could be heard over urban noise (see Urban nightingales' songs are illegally loud).
Now, researchers have shown this adaptation is happening on a population level, as well in cities around Europe.
Hans Slabbekoorn and Ardie den Boer-Visser, at Leiden University in the Netherlands, recorded and compared great tits (Parus major) singing in 10 European cities and in nearby forests.
They found that in all the cities, songs were sung faster and in higher pitches than in nearby forests. (Listen to recordings from Brussels and the nearby Rivière forest.) Slabbekoorn says the differences between the urban and rural songs are "remarkably" consistent across all the sites surveyed.
The researchers say this is explained by the fact that urban noise pollution, most of which comes from traffic, tends to be at a lower pitch. This drowns out low-pitched birdsong notes. In contrast, noise in natural environments is not biased towards one end of the frequency spectrum. (Listen to an isolated song, the same song in a noisy urban setting, and in a forest setting.)
Another factor contributing to the high-pitched and faster urban songs is the relative openness of city landscapes compared to forests. Earlier work showed that songs in forested habitats were sung lower and more slowly than those in open countryside, because these songs are less likely to be lost in reflections in the dense foliage.
Exactly how city birds adopt a higher pitched repertoire is not understood.
One theory is based on the fact that great tits are known to learn songs from their neighbours. It could be that young birds simply do not hear the low notes produced by other birds and drop them out of their song. However, this would imply that urban songs generally had fewer notes than forest songs, which was not the case.
Slabbekoorn points out that songs with low notes may be dropped in their entirety, leaving the birds with an exclusively high-pitched repertoire.
Another theory is that the birds are forced to use higher-pitched songs because they find the low-pitched songs do not prompt the feedback they desire from other birds. The songs are either used for attracting mates or defending territory.
The urban environment could be exerting evolutionary pressures on the bird songs, meaning that birds with a higher-pitched repertoire are more successful at mating. Slabbekoorn thinks the cause is likely to be a combination of several mechanisms.
Nature or nurture?
"Understanding just how the changes come about is very puzzling," says Peter Marler, an expert on neurobiology and behaviour at the University of California Davis in the US. "We really don't know much about song development in great tits. There have been very few lab studies on this species."
And lab studies are exactly what Slabbekoorn is planning next. In an attempt to find the underlying mechanism, he and his colleagues will put birds that are used to quiet environments in noisy conditions and see how they manage.
So is it nature or nurture at work? The clear response of the great tits to changes in their environment favours the nurture factor, but that is complicated by the fact that the changes in birdsong could then be transmitted to the next generations via genes.
"The study is very compelling, and generates a lifetime of hypotheses to explore," says Marler.
Journal reference: Current Biology (vol 16, p 2326)